41 lines
679 B
C++
41 lines
679 B
C++
#include"stdio.h"
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#include"math.h"
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double jie(int n)
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{
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double y=1;
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int i;
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for(i=2;i<=n;i++)
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y*=i;
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return y;
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}
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double c(int n,int x)
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{
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double z;
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z=jie(n)/jie(x)/jie(n-x);
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return z;
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}
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int main()
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{
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double ans;
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int n;
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int s;
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double x,y;
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int i;
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while(scanf("%d%d",&n,&s)!=-1)
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{
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if(s>n)
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{
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printf("100.00000\n");
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continue;
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}
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x=y=0;
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for(i=0;i<s;i++)
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x+=c(n,i)*pow(2,n-i);
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for(i=0;i<=s;i++)
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y+=c(n,i)*pow(2,n-i);
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ans=x/y;
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printf("%0.5lf\n",ans*100);
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}
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return 0;
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}
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